feat(GlDuoChat): Update the Duo Chat appearance

What does this MR do?

This UI MR is a preparation before adding suggestions.

• Remove gray background to make the interface lighter. We usually don't use gray background for drawers and modals.
• Remove header and footer borders to make the surface more solid. Now it's a UI change but it will make more sense with suggestions.
• Increase boder radius for messages and controls to distinguish the rest of GitLab UI from the Duo Chat. Because the Duo Chat is a feature on top of GitLab. So, it feels more natural when it looks more like a chat and less like a GitLab form.
• Add shadow to distinguish the Duo Chat more from the rest of the UI

Screenshots or screen recordings

before	after

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

gitlab#474922 (closed)